Re: [Clarification] writes to kernel addresses that came from userspace

On Mon, Sep 13, 2021 at 09:59:36AM +0200, Bernd Petrovitsch wrote:

Hi all!

On 12/09/2021 18:20, Len Baker wrote:
[...]

quoted

[extracted from the KSPP]

It should be possible to perform taint tracking of addresses in the kernel
to avoid flaws of the form:

copy_from_user(object, src, ...);
...
memcpy(object.address, something, ...);

[end of extracted]

My question is: Why is this scenario a flaw?

If I understand correctly, the copy_from_user() function copies n bytes of
src (in user space address) to object (in kernel space address). I think > that it is the correct way to act. Then, in kernel space the object is

Yup.

quoted

modified. So, I don't see the problem. Sorry if it is a trivial question
but I can not figure it out on my own.

Shouldn't the memcpy() be a copy_to_user() as object.address is setup by the
user space and thus a user space address?

Right, _correct_ code would pass a userspace address, and use
copy_to_user() for writing to it. The goal here would be to find the
kinds of paths that might lead to bad conditions (i.e. answering "is it
possible for a userspace-controlled value to reach a place in the kernel
that didn't sanity-check it before doing indexing, sizing, etc?").

-- 
Kees Cook

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